Nuclear magnetic resonance (NMR) is a phenomenon exhibited by a select group of atomic nuclei and is based upon the existence of nuclear magnetic moments in these nuclei (termed "gyromagnetic" nuclei). When a gyromagnetic nucleus is placed in a strong, uniform and steady magnetic field (a so-called "Zeeman field") and perturbed by means of a weak radio-frequency (RF) magnetic field, it precesses at a natural resonance frequency known as a Larmor frequency, which is characteristic of each nuclear type and is dependent on the applied field strength in the location of the nucleus. Typical gyromagnetic nuclei include .sup.1 H (protons), .sup.13 C, .sup.19 F and .sup.31 P. The resonant frequencies of the nuclei can be observed by monitoring the transverse magnetization which results after a strong RF pulse. It is common practice to convert the measured signal to a frequency spectrum by means of Fourier transformation.
Although identical nuclei have the same frequency dependence upon the magnetic field, differences in the chemical environment of each nucleus can modify the applied magnetic field in the local vicinity of the nucleus, so that nuclei in the same sample do not experience the same net magnetic field. The differences in the local magnetic field result in spectral shifts in the Larmor frequencies between two such chemically non-equivalent nuclei, called "chemical shifts". These chemical shifts are interesting in that they reveal information regarding the number and placement of the atoms in a molecule and in the positioning of adjacent molecules with respect to each other in a compound.
Unfortunately, it is not always possible to interpret the frequency spectra produced by the chemical shifts because of other interfering and dominant interactions
This is particularly true in NMR spectroscopy of solids. In liquid NMR spectroscopy, the rapid motion of the liquid molecules tends to isolate the nuclei and separate the nuclear interactions, so that it is easier to distinguish separate nuclei in the final output. In solid state NMR, there are many interactions between the molecules which obscure the output. For example, the magnetic moments in neighboring nuclei perturb each other, resulting in interactions called dipole-dipole couplings. These couplings tend to broaden the characteristic resonance peaks and obscure the "fine" resonant structure produced by the chemical shifts. An additional problem found in solids, which is not present in liquids, is that the orientation of the solid molecules is relatively fixed with respect to the applied Zeeman field and, accordingly, the chemical shifts are anisotropic, in that a component of the resonant frequency depends on the physical orientation of the molecules with respect to the applied field.
Therefore, it is essential to suppress some interactions over others to obtain a meaningful output. This is usually done by perturbing the system at selected frequencies to cause unwanted interactions to cancel or average to a reduced amplitude. For example, in solids, the aforementioned chemical shift anisotropy is usually greatly reduced by orienting the solid sample at a "magic angle" (54.degree.44') with respect to the applied Zeeman field and physically rotating the solid at a relatively rapid rate causing the anisotropic field components to average to zero.
Similarly, by well known techniques, it is possible to reduce the unwanted spin-spin interactions by irradiating the nuclei with additional pulses of RF energy at or near the Larmor frequencies. By properly selecting various orientations and phases of the RF pulses, the polarization of the perturbing nuclear spin systems in neighboring groups can be changed, effectively averaging out the spin interactions so that the contribution to the final output is greatly diminished. Since the Larmor frequencies for each nuclear type are distinct, an applied RF frequency will have a much greater effect on those nuclei which have a Larmor frequency which is close to the applied frequency than those nuclei in which the Larmor frequency is considerably different. Thus, the applied RF fields can be used to affect one type of nucleus while leaving others unchanged.
A conventional NMR technique which is used obtain NMR spectra of organic solids involves obtaining an output from .sup.13 C nuclei in the solid of interest. However, the .sup.13 C nuclei are not directly excited. Instead, .sup.1 H nuclei (protons) in the test sample are excited and various RF pulse sequences are used to transfer polarization from the protons to the .sup.13 C nuclei which are then observed. This technique is carried out in the presence of magic angle spinning and is referred to as Cross Polarized Magic Angle Spinning or CPMAS. The technique is described in more detail in "Solid State NMR for Chemists", Colin A. Fyfe, CFC Press, P.O. Box 1720, Guelph, Ontario, Canada N1H 6Z9, Chapter 6 and "High Resolution Solid State NMR of Carbon-13", R. G. Griffin, Analytical Chemistry, v. 49, p. 951A (1977) and generally involves performing a process or "experiment" in the time domain, consisting of three time intervals. During the first time period, the sample protons are placed in a excited, coherent non equilibrium state by the application of a single RF pulse to the sample, or by a sequence of RF pulses.
Next, the sample is subject to cross polarization during which RF pulses or continuous RF energy is applied to both the protons and the .sup.13 C nuclei, which causes the transfer of coherence or polarization from the protons to the .sup.13 C nuclei under observation.
The cross polarization period is followed by data acquisition in which the cross-polarizing energy is removed and a free induction decay (FID) occurs in the .sup.13 C nuclei and the resonance frequencies of the .sup.13 C nuclei are measured. During this period it is conventional to apply further pulses or continuous wave RF energy to the protons in order to decouple or prevent further interaction of the protons and .sup.13 C nuclei.
For many solid materials, the CPMAS technique produces well-differentiated spectra of the carbon nuclei in the test sample with good chemical shift resolution. The CPMAS spectra appear very similar to .sup.13 C NMR spectra of samples in solution, and they are used in much the same way.
However, in some cases, it is convenient to suppress some information in favor of other information. For example, the carbon atoms in a particular sample may be arranged so that resonance peaks or lines overlap and obscure each other. Alternatively, it may be desirable to differentiate the carbon resonance lines or spins, for example, by the number of directly bonded protons. In this manner, additional information can be gathered about the molecular structure.
The carbon spins typically can be differentiated by means of various spectral "editing" techniques. In .sup.13 C NMR spectroscopy of liquid samples, various well known editing techniques are commonly used to differentiate carbon spins according to the number of directly bonded protons. An example of such a technique is the so called DEPT RF pulse sequence described in "Proton-polarization Transfer Enhancement of a Heteronuclear Spin Multiplet with Preservation of Phase Coherency and Relative Component Intensities", D. T. Pegg, D. M. Doddrell, M. R. Bendall, Journal of Chemical Physics, v. 77, n. 6, pp. 2745-2752 (1982). The DEPT technique can differentiate between resonances resulting from CH, CH.sub.2 and CH.sub.3 functional groups.
However, only a few prior art spectral editing methods have been used for solid state CPMAS experiments; one common such prior art editing technique is called dipolar dephasing and is described in "Selection of Non-Protonated Carbon Resonances in Solid State Nuclear Magnetic Resonance", S. J. Opella, D. M. H. Frey, Journal of the American Chemical Society, v. 101, p. 5855 (1979). In this editing method, the .sup.13 C proton decoupling energy is interrupted briefly just before the start of data acquisition. The result is that the carbon spins which have one or more attached protons are suppressed in the output, while non protonated carbons are largely unaffected. This technique is useful, but it does not differentiate between the carbons that have one or more attached protons (CH, CH.sub.2 and CH.sub.3 groups).
Other, so called "difference" prior art spectral editing techniques have been proposed as described in "Quantitative Determination of the Concentrations of .sup.13 C-.sup.15 N Chemical Bonds by Double Cross-Polarization NMR", J. Schaefer, E. O. Stejskal, J. R. Garbow and R. A. McKay, Journal of Magnetic Resonance, V. 59, pp. 150-156 (1984) and "Magic Angle Sample Spinning NMR Difference Spectroscopy", H. J. M. DeGroot, V. Copie, S. O. Smith, P. J. Allen, C. Winkle, J. Lugtenburg, J. Herzfeld and R. G. Griffin, Journal of Magnetic Resonance, v. 77, pp. 251-257 (1988), but they typically depend on either the presence of a third nuclear species or on isotopic enrichment and labeling, and are therefore useful only in special cases.
Accordingly, it is an object of the present invention to provide editing of conventional CPMAS solid state NMR spectra
It is another object of the present invention to increase the number of carbon sites which can be resolved in conventional CPMAS solid state NMR spectra.
It is still another object of the present invention to provide editing of conventional CPMAS solid state NMR spectra by modifying the proton decoupling portion of a conventional CPMAS solid state NMR experiment
It is yet another object of the present invention to provide a editing of conventional CPMAS solid state NMR spectra by modifying the proton decoupling energy during the acquisition of data.
It is a further object of the present invention to provide a new RF pulse sequences during the cross-polarization period to effectively suppress carbon resonances other than methylene carbons.